The invention relates to a cathode-ray tube and more particularly, but not exclusively, to a television camera tube.
A television camera tube disclosed in U.S. Pat. No. 3,912,851 comprises a tubular envelope portion of insulating material having an internally provided electrically conductive wall coating and at least one electrode extending transversely to the wall coating, the electrode being supported in the envelope portion by a supporting surface extending transversely to the longitudinal axis of the envelope portion, and the supporting surface being formed by an envelope portion whose internal transverse dimensions decrease in a substantially stepwise manner. In this case the gauze electrode bears on a shoulder formed by a local restriction of the envelope and is secured to the tube wall by means of indium.
U.S. Pat. No. 2,938,134 discloses an electron gun system of which a number of electrodes are supported in an envelope by supporting surfaces which have been obtained by a stepwise narrowing of the inside diameter of the envelope. The electrodes are urged against the supporting surfaces by resilient means.
Netherlands Patent Specification No. 42,114, to which British Pat. No. 431,246 corresponds, discloses a cathode-ray tube in which the electrodes are placed in a cylindrical insulating member provided inside the tube. The insulation member comprises steps to which the electrodes having a resilient edge are clamped. A part of the inner wall of the insulation member is coated with an electrically conductive layer.
The present development of cathode ray tubes and in particular that of television camera tubes is directed more and more to the manufacture of small tubes subjected to narrow tolerances. This development is associated with a simplification of the tube construction in particular as regards the construction of the electrode system used in the tube. If possible, the electrodes are replaced by wall electrodes in the form of thin-film electrodes provided on the inner wall of the envelope of the tube. However, a problem is that the necessary interruptions in the conductive wall coating, to obtain wall electrodes which are electrically insulated from each other, may cause a local disturbance of the electric field distribution in the tube. Such a disturbance is caused mainly by electric charge of the tube wall at the area of an interruption in the conductive wall coating. The influence of such a disturbance of the electric field distribution on, for example, the path of rays of an electron beam generated in the tube is more disturbing as the interruption is less rotationally symmetrical and as the inside diameter of the tube envelope is smaller. Field disturbances may furthermore be caused by the connections with which, for example, gauze electrodes and electrodes to limit the diameter of an electron beam, for example a diaphragm, are connected to a conductive wall coating.